Materials Research for Nuclear Fuels

The 2011-Fukushima Daiichi accident emphasized how essential it is to have a thorough understanding of the reaction of nuclear fuel materials with water. It is unlikely that a similar, earthquake related accident would occur in Sweden. Yet the reaction of nuclear fuel with water is important from a different perspective: spent nuclear fuel from Swedish reactors will be stored in deep underground repositories, which appears as a promising solution [1] to the problem: what to do with nuclear waste?

The repository’s implementation requires a comprehensive understanding of fuel corrosion processes in order to present a safety case based on scientifically sound estimations of possible environmental impacts. After a long time, the spent nuclear fuel – composed primarily of uranium dioxide with other actinides (Np, Pu) and fission products – will react with ground water. In collaboration with Svensk kärnbränslehantering (SKB) the group of Peter Oppeneer performs materials modeling simulations to investigate the dissolution of the nuclear fuel through reaction with water. We use ab initio molecular dynamics and atomistic thermodynamics to simulate the reactivity of UOsurfaces with water, which furnish the conclusion that UOsurfaces will always react with water under equilibrium conditions (atmospheric pressure and room temperature) leading to its dissolution in water [2], see figure 1.

Dissolution phase diagram
Figure 1. Ab initio computed temperature versus water pressure phase diagram of water molecules adsorption on the (111) UO2 surface. The blue line indicates the range of water partial pressures at which the molecule desorbs at room temperature [2].

The in-operando behavior of nuclear fuel in a reactor is a complex phenomenon that is influenced by a large number of material properties, which include thermo-mechanical strength, chemical stability, microstructure, and defects. As a consequence, a comprehensive understanding of the fuel material behavior presents a significant modeling challenge, which must be mastered to improve the efficiency and reliability of nuclear reactors. It is also essential to the development of advanced fuel materials for next-generation reactors. In collaboration with SKB we have investigated the influence of fission defect such as He, Xe, or oxygen and uranium vacancies on the thermo-mechanical stability of the UOwhich is a crucial factor to increase nuclear fuel burn-up and thus to improve fuel efficiency [3].

In collaboration with the EU-JRC (Karlsruhe) we computational investigate the thermal conductivity of actinide dioxides, which is an important quantity for improving reactor efficiency. We use supercell simulations to compute the lattice dynamics and thermal properties of NpOand UO2; our simulations highlight the importance of high-energy optical phonons to the ensuing heat transport [4] (see figure 2).

NpO2 phonons
Figure 2. Left panel: phonon linewidth distribution calculated along the high-symmetry lines for fcc NpO2 at 300 K. Different colors in the linewidth distribution indicate different phonon branches, and the q-dependent linewidth is depicted by the width of the branch. Right panel: accumulated phonon thermal conductivity κ calculated as a function of the phonon energy, and its derivative [4].

Contact

Pablo MaldonadoPeter Oppeneer

Funding

Svensk kärnbränslehantering AB (SKB), EU-Euratom-FP7 “REDUPP”, EU-JRC Karlsruhe.

References

  1. F.N. von Hippel, R.C. Ewing, R. Garwin and A. Macfarlane, Time to Bury Plutonium. Nature 485, 167−168 (2012).
  2. P. Maldonado, L. Evins and P.M. Oppeneer, Ab Initio Atomistic Thermodynamics of Water Reacting with Uranium Dioxide Surfaces. J. Phys. Chem. C 118, 8491 (2014).
  3. Y. Yun and P.M. Oppeneer, Ab initio Design of Next-Generation Nuclear Fuels. MRS-Bull. 36, 178 (2011).
  4. P. Maldonado et al., Crystal dynamics and thermal properties of neptunium dioxide. Phys. Rev. B. 93, 144301 (2016).